1. Field of the Invention
A useful method of applying inhibitors to producing oil wells is to displace or "squeeze" them into the producing reservoir. In the case of a corrosion inhibitor, the inhibitor provides corrosion protection to the well as the inhibitor is produced back from the formation. When the returning corrosion inhibitor concentration reaches a low level (typically 1-10 ppm), the formation must be "resqueezed."
There presently exists no analytical method, either field or laboratory, for determining inhibitor concentrations in produced fluids (i.e. oil and water) at the low levels required. In the case of corrosion inhibitors, reliance is now placed in corrosion monitoring techniques to establish when a well must be retreated. The obvious difficulty in this use of corrosion monitoring is that the well must corrode before it again receives a corrosion prevention treatment.
Returning corrosion inhibitor partitions between the liquid phases of the fluid produced from a well. The desired analytical method (or methods) thus needs to be suitable for determination of inhibitor in brines, crude oils, and condensates. What is needed is the total returning corrosion inhibitor concentration.
The corrosion inhibitor chemistry of most general interest for oil field use is the chemistry of nitrogen-containing polar organic compounds. Inhibitors are usually made from commercial intermediates that are not pure compounds. Nonetheless, the species of interest is usually one or more of the following:
1. Amines and Diamines-primary, secondary, tertiary and quaternary. PA0 2. Amides. PA0 3. Amino-amides. PA0 4. Amine Salts. PA0 5. Imidazolines.
The side chains are typically 1 to 18 CH.sub.2 units long and may contain double bonds. The diamines are most frequently ethylene or propylene diamines. The final inhibitor or its precursors are sometimes ethoxylated.
The analytical method presently used in the industry for determining corrosion inhibitor content is an ion pair technique suitable for measuring corrosion inhibitors in water with a detection limit of about 5 ppm. The essence of the technique is the addition of an excess of a large anionic molecule to the water containing the cationic corrosion inhibitor. The ion pair formed is then extracted into a solvent and its concentration determined colorometrically.
The first obvious limitation of the ion pair technique is that it detects all large cationic molecules, i.e., it is neither specific for nor all inclusive for corrosion inhibitors. The second difficulty is that the method is useful for aqueous systems only. To determine inhibitor concentration in the oil phase, the inhibitor must be extracted from the oil into water before the ion pair technique can be used. Not only is the extraction difficult, but many other crude oil components are also extracted and therefore interfere with the final determinations.
2. The Prior Art
U.S. Pat. No. 4,665,981 issued to Hayatdavoudi relates to a method and apparatus for inhibiting corrosion of well tubing by monitoring the concentration of corrosive element therein with a conventional transducer and using a computer system to optimize the rate of injection of said inhibitor responsive to the monitored condition.
U.S. Pat. No. 3,298,438 issued to Anthony et al. discloses a method for preventing corrosion of metallic surfaces that are exposed to corrosive vapors by conducting analytical measurements using chromatographs specially designed for measuring the inhibitor and the acid gas content.